1. Field of the Invention
The present invention relates to a system for driving a solenoid valve for an internal combustion engine. Particularly, it is directed to a system for driving a solenoid valve for an internal combustion engine which system controls a drive circuit for a fuel injecting solenoid valve with a pulse signal.
2. Description of the Prior Art
Heretofore, systems which control a drive circuit for a solenoid valve for an internal combustion engine with a DC signal have generally been well known. However, these conventional systems have the disadvantage in that the loss of power is increased.
In view of the above, systems have been proposed which control a drive circuit for a solenoid valve for an internal combustion engine with a pulse signal as shown, for example, in Japanese Patent Laid-Open Publication No. 203830/82. This system is advantageous in that the consumption of power can be reduced in comparison with the above conventional system which employs a DC signal for controlling the drive circuit.
The amount of fuel injected depends on the duration (hereinafter referred to as "injector ON time") of the opening of the solenoid valve since the opening degree of the solenoid valve and the fuel pressure are constant. It is also generally well known that the injector ON time varies according to the operating conditions of an engine. For example, at the time of acceleration it is necessary to make the injector ON time relatively large.
The operating state of the engine is determined on the basis of, for example, engine speed, pressure (intake manifold pressure) in the intake manifold, engine coolant temperature (engine temperature) and atmospheric pressure.
In the drive control system using a pulse signal, as shown in FIGS. 5(a) and 5(b), the injector ON time and hence an output time Ti of an injector ON control signal fed to a drive circuit, which determines the injector ON time, is divided into a shortest time Tomin required for lifting a solenoid valve and a holding time Thold for holding the solenoid valve in a lifted state.
More specifically, the shortest time Tomin required for lifting the solenoid valve is a single pulse width time, while the solenoid valve holding time Thold is the total time of plural pulse signals whose period T is determined by a monostable multivibrator for example.
A solenoid valve holding current is predetermined according to the characteristics of the solenoid valve. Therefore, the duty ratio of a pulse signal of the holding time Thold fed to a solenoid valve driving circuit is also determined in advance.
The above conventional techniques involve the following problems.
As previously noted, the output time Ti of the injector ON control signal is determined according to an operating state of the engine, and the shortest time Tomin required for lifting the solenoid valve at the output time Ti of the injector ON control signal is predetermined according to characteristics of the solenoid valve.
Therefore, the solenoid valve holding time Thold is determined as a time corresponding to a difference obtained by subtracting the shortest time Tomin required for lifting the solenoid valve from the output time Ti of the injector ON time control signal.
However, as previously noted, where the period of pulse signal at the solenoid valve holding time Thold is decided in advance, if the output time Ti of the injector 0N control signal becomes shorter or longer as indicated at T'i, than the time Ti shown in FIG. 5(a), as shown in FIG. 5(d), the solenoid valve holding pulse signals in the solenoid valve holding time are not an integer multiple of a certain period and this results in a remainder time Tr [see FIGS. 5(e)].
As a result, the waveform of the solenoid valve holding circuit signal assumes the state of FIG. 5(f) relative to the state (c) of the injector ON control signal (b). That is, the solenoid valve holding current value, upon the lapse of the output time of the injector ON signal, differs depending on whether the remainder time Tr is present or not. Consequently, according to the conventional drive control system using a pulse signal, there arises a difference in the duration from after the lapse of the output time of the injector ON control signal until the solenoid valve actually assumes a closed state. Thus, the injector 0N time of the solenoid cannot be properly controlled.